cmd/dist: when -compile-only is set, write binaries to os.DevNull

Previously, we were writing binaries to GOROOT without ever reading them.

Updates #28387

Change-Id: Ic84999f46a367fcbf2ae1eac8bfdcbceb24dc748
Reviewed-on: https://go-review.googlesource.com/c/go/+/207342
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/test.go

@@ -748,7 +748,7 @@ func (t *tester) registerTests() {
 			heading: "API check",
 			fn: func(dt *distTest) error {
 				if t.compileOnly {
-					t.addCmd(dt, "src", "go", "build", filepath.Join(goroot, "src/cmd/api/run.go"))
+					t.addCmd(dt, "src", "go", "build", "-o", os.DevNull, filepath.Join(goroot, "src/cmd/api/run.go"))
 					return nil
 				}
 				t.addCmd(dt, "src", "go", "run", filepath.Join(goroot, "src/cmd/api/run.go"))